1. Field of the Invention
This invention generally relates to Class D amplifiers. More particularly, this invention relates to a circuit and a method for suppressing pop and click noise during power-on and power-off of the amplifier.
2. Description of Related Art
The major components of a basic class D amplifier are shown in FIG. 1. The raw input 1 is connected to an adder circuit 4, which subtracts from the input 1 a feedback signal 5. The basic amplifier uses feedback from the output of a half-bridge output stage 19 to help compensate for variations in the bus voltages and to correct distortion. The half-bridge output 19 is connected to a feedback path or network 2, whose output 5 is then combined with the raw input 1 at the adder 4. The adder output 6 then is connected to a filter 3, which generates a modified audio input 14. The comparator, C, 11 compares a high frequency triangular wave 15 and the modified audio input 14 and generates pulses whose width corresponds to the amplitude and frequency of the audio input signal 14. The pulse width modulated (PWM) output 18 is fed into the speaker 16. The output 17 of the comparator drives the switching controller 12. The switching controller 12 drives a high power switch 19, whose output 18 is a higher power replica of the comparator's output 17.
When audio circuits are turned on, pop and click noise can occur on the audio speaker output as quiescent voltages are established. Such effects are undesirable and measures must be taken to suppress them. In the case of Class D amplifiers, there is a feedback loop which includes the nodes which are connected directly to the audio speaker output. There is the problem of establishing the operating point of the feedback loop without causing a step on the output which would lead to unwanted audio artifacts.
The following patents represent the prior related art.    U.S. Pat. No. 7,528,653 B2—Class-D Amplifier with Noise Immunity Feedback (May) describes a class-D amplifier which includes a switching transistor section, a filter, an analog to digital converter, and a feedback module. This amplifier structure provides a method and an apparatus for dynamically minimizing and/or compensating for total harmonic distortion (THD), transient noise, and/or power supply noise of class-D amplifiers.    U.S. Pat. No. 7,432,759 B2—Elimination of Pop-Up Noise in Class D Amplifier (Cho et al.) describes a class D amplifier which includes a driver circuit and a reset circuit. The driver circuit is configured to amplify a PWM (pulse width modulation) signal to generate an amplified PWM signal. The reset circuit applies a predetermined voltage at an input of the driver circuit for a time period after a power supply voltage is applied or before the power supply voltage is deactivated, for eliminating pop-up noise.    U.S. Pat. No. 7,417,504 B2—Startup and Shutdown Click Noise Elimination for Class D Amplifier (Strydom et al.) describes a method of minimizing an audible click noise from a speaker in a Class D audio power amplifier upon shutdown or startup of a switching stage, having two switches series connected at a node, the speaker connected to the node via an output filter comprising an inductor and a capacitor, the switches being controlled by a controller.    U.S. Pat. No. 7,394,314 B2—Class-D Amplifier with Noise-Immunity Feedback (May) describes a class-D amplifier which includes a switching transistor section, a filter, an analog to digital converter, and a feedback module. This amplifier structure provides a method and an apparatus for dynamically minimizing and/or compensating for total harmonic distortion (THD), transient noise, and/or power supply noise of class-D amplifiers.